This study will test the theory that IDU's may have higher expression of iNOS than controls, which may explain the observed association of a Nitric Oxide resistant strain with IDU's. Initially 10 HIV-negative IDU volunteers will be recruited from outpatient clinics at New York and Hospitals. They will be compared to 10 age, sex and smoking history- matched non-IDU HIV negative controls. Blood samples will be tested for iNOS expression and evidence of Nitric Oxide production.